Polylactic acid decomposes as a result of the action of microbes and enzymes; that is, it is biodegradable. It is converted into lactic acid or carbon dioxide and water that are harmless to the human body. Thus, polylactic acid has attracted attention as an alternative to medical materials and general-purpose resins. Although polylactic acid is a crystalline resin, it is crystallized at a low rate, and it exhibits features practically similar to those of noncrystalline resins. Namely, polylactic acid is softened rapidly and extremely at about the glass transition temperature (typically, less than 1/100 in terms of elastic modulus), and thus, it has been difficult to attain sufficient properties in terms of heat resistance, moldability, mold releasability, and the like.
As a measure of overcoming such disadvantages, JP Patent Publication (Kokai) No. 9-278991 (Patent Document 1) discloses a method for producing an aliphatic polyester molded product, wherein an aliphatic polyester composition comprising a aliphatic polyester, which does not produce a crystal upon a drop of temperature from the melting point to the glass transition temperature at a rate of 10° C./min, and at least one transparent nucleating agent selected from the group of compounds consisting of an aliphatic carboxylic acid amide, an aliphatic carboxylate, an aliphatic alcohol, and an aliphatic carboxylic acid ester having a melting temperature of 40° C. to 300° C. is molded, and the aliphatic polyester compound is thermally treated during or after the molding. Examples of aliphatic carboxylic acid amides include aliphatic monocarboxylic acid amides, N-substituted aliphatic monocarboxylic acid amides, aliphatic biscarboxylic acid amides, N-substituted aliphatic carboxylic bisamides, and N-substituted ureas.
JP Patent Publication (Kokai) No. 9-278991, however, does not describe stereocomplex crystals. The crystallinity of the aliphatic polyester molded article disclosed in this publication is substantially homogeneous, and improvement in heat resistance and crystallization speed is not yet sufficient.
JP Patent Publication (Kokai) No. 2003-128900 (Patent Document 2) discloses the use of a polylactic acid stereocomplex obtained by mixing poly-L-lactic acid (PLLA) and poly-D-lactic acid (PDLA) in a molten state. This publication describes that the polylactic acid stereocomplex has a high melting point and a high crystallinity and that molded articles excellent in heat resistance can be obtained.
JP Patent Publication (Kokai) No. 2003-192884 (Patent Document 3) discloses a polylactic acid-based polymer composition comprising 100 parts by weight of a polymer (A) capable of generating a stereocomplex that is composed mainly of a polylactic acid comprising a poly-L-lactic acid composed mainly of L-lactic acid and a poly-D-lactic acid composed mainly of D-lactic acid, and 0.01 to 5.0 parts by weight of a metal phosphate (B) as a nucleating agent for crystallization.
Use of the polymer composition described in JP Patent Publication (Kokai) No. 2003-128900 or JP Patent Publication (Kokai) No. 2003-192884, however, results in an insufficient ratio of stereocomplex crystals of poly-L-lactic acid and poly-D-lactic acid. Thus, improvement in heat resistance of the resulting sterocomplex and in the crystallization speed thereof was insufficient.
Patent Document 1: JP Patent Publication (Kokai) No. 9-278991
Patent Document 2: JP Patent Publication (Kokai) No. 2003-128900
Patent Document 3: JP Patent Publication (Kokai) No. 2003-192884